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C. L. O'Neill, R. J. Medina, A. D. Bhatwadekar, T. A. Gardiner, A. W. Stitt; Characterisation and Role of Defined Endothelial Progenitor Cell Sub-Populations in Retinal Angiogenesis. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4386.
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© ARVO (1962-2015); The Authors (2016-present)
Previous studies have indicated that endothelial progenitor cells (EPCs) play an important role in retinal vascular repair and angiogenesis. However, controversy surrounds the heterogeneous nature of EPCs used for in vitro and in vivo studies and the distinct lineages that transplanted cells can adopt. The aim of this study is to identify the precise phenotype of EPC sub-populations and examine their relative interactions with retinal microvascular endothelial cells (RMECs).
EPCs were isolated from human peripheral blood by culturing the mononuclear cell fraction on fibronectin or collagen. EPCs were fully characterised by flow cytometry and immunocytochemistry. Clonogenic and proliferative potential and ability to induce RMECs sprout formation was studied. EPCs were labelled using Qdot nanotechnology and then co-incubated with RMEC to determine cell-cell interactions during vascular tube formation.
The two different isolation protocols resulted in distinct EPC populations which could be classified as early EPCs (eEPCs) and so-called outgrowth endothelial cells (OECs). eEPCs appeared at 1 week as spindle shaped cells which did not proliferate in vitro. They expressed endothelial markers CD31 and VEGFR2 but also retained the haematopoietic markers CD34 and CD14. By contrast OECs appeared at 4 weeks as cobblestone shaped, highly proliferative cells. OECs represented a very homogeneous population characterised as CD31+, VEGFR2+, CD105+, CD90+, CD29+, Vim+, STRO-1-, CD45-, CD14-, CD34. Both eEPCs and OECs induced significant RMEC vascular tube formation in co-culture. Only the OECs incorporated into RMEC sprouts while the eEPCs remained adjacent to but separate from the "resident" endothelial cells.
This study provides a full phenotypical analysis of both subsets of EPCs. Functional analysis has revealed that although both eEPCs and OECs stimulate retinal angiogenesis in vitro only the later directly interact with the resident endothelium. These findings add to our understanding of the role of various EPC sub-populations in the retina and have implications for harnessing these cells in retinal microvasculopathies.
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